Double Down Achievement in Halo: The Master Chief Collection

In Multiplayer, the Double Kill medal is awarded to players who kill two opponents within four seconds of each other. As such, you will want to focus your attention on playing offensively when working on this achievement, utilizing close-quarters combat and weaponry to kill enemies quickly.

As a suggestion, this is best done when boosted with two other players in a Custom Game on a small map such as Warlord. All three players should meet up in a central location on the map. Then one player should kill the other two players quickly to receive their Double Kill medal and go on to repeat the process a second time for the achievement.